Sacroiliac Joint (SIJ)

The complex joint known as the sacroiliac (SI) joint connects the ilium, one of the pelvic bones, to the sacrum, the triangular bone at the base of the spine. 

Understanding the sacroiliac joint's anatomy is essential to comprehend how it works and the conditions that can affect it. Let's examine its structure and makeup in more detail.

Bones:

The articulation of the auricular surfaces of the sacrum and ilium results in the formation of the sacroiliac joint. 

Rough, asymmetrical regions with fibrocartilage cover the auricular surfaces. The synovial portion of the sacrum and the ilium together make up the majority of the joint.

Ligaments:

The sacroiliac joint is stabilized and supported by several ligaments. These consist of:

• Anterior sacroiliac ligament: This powerful ligament strengthens the front of the joint by joining the anterior surface of the sacrum to the iliac fossa.

• Posterior sacroiliac ligament: This ligament supports the back of the joint by joining the ilium to the posterior surface of the sacrum.

• Interosseous sacroiliac ligament: It is a short, strong ligament located deep within the joint. It fills the gap between the sacrum and ilium, strengthening the joint.

• Sacrotuberous ligament: This broad ligament extends from the lower sacrum and sacroiliac joint to the ischial tuberosity. It helps stabilize the joint and plays a role in transferring forces from the trunk to the lower extremities.

• Sacrospinous ligament: Situated inferior to the sacrotuberous ligament, it connects the lower sacrum to the ischial spine. It assists in providing stability to the joint.

Joint:

A synovial joint with a synovial membrane and synovial fluid, the sacroiliac joint is categorized as a diarthrodial joint. The lining of the joint cavity by the synovial membrane results in the production of synovial fluid, which lubricates the joint and feeds the articular surfaces.

Muscles:

The sacroiliac joint is surrounded by several muscles that help to support it and control movement. These muscles include the piriformis, multifidus, erector spinae, gluteus maximus, and deep hip rotators.

Nerve Supply:

The superior, inferior, and posterior femoral cutaneous nerves are some of the lumbosacral plexus branches that innervate the sacroiliac joint. These nerves transmit sensory data and aid in the motor control of the muscles in the immediate area.

Movements:

In comparison to other joints in the body, the sacroiliac (SI) joint has relatively few movements. Small movements at the joint are necessary for everyday activities like walking, running, and standing. The sacroiliac joint can move in the following ways:

Nutation: 

The anterior tilt or forward movement of the sacrum in relation to the ilium is referred to as nutation. The sacrum moves inferiorly and anteriorly during nutation, bringing the base of the sacrum closer to the pubic bone.

Counternutation: 

The sacrum's posterior tilt or backward movement is referred to as counternutation, which is the opposite motion of nutation. The sacrum moves superiorly and posteriorly, away from the pubic bone, during counternutation.

Glide: 

Small gliding movements between the sacrum and ilium are made possible by the sacroiliac joint. When performing activities like walking or bearing weight, the joint is subjected to forces and stresses that cause these gliding movements to happen.

Rotation: 

The sacroiliac joint also permits a small amount of rotational movement. This rotational movement causes a slight torsion of the joint as the sacrum rotates on an axis. The range of rotation varies from person to person and is not very wide.

Pathology:

Sacroiliac joint dysfunction or pathology can impede normal movement and cause low back pain. 

Trauma

Inflammation

Joint degeneration

Sacroiliac joint dysfunction

Sacroiliitis

Sacroiliac joint syndrome

Hypermobility

Special tests:

1. Fortin finger test
2. Gillette test
3. Posterior distraction test
4. Sacroiliac distraction test
5. Sacral thrust test
6. Rotational stress test
7. Long sit test
8. Gaenslen's test
9. Faber's test
10. Leg length test

In order to diagnose and treat associated conditions, it is crucial to comprehend the sacroiliac joint's anatomy. X-rays, CT scans, and MRI imaging procedures can give you more details about the structure of the joint and, if necessary, can help you decide how to proceed with medical interventions. For an accurate diagnosis and suitable treatment of problems related to the sacroiliac joint, it is crucial to seek medical advice.